Batteries are the power source of choice for many sensor systems. Maintenance, replacement, and disposal of batteries are expensive, time consuming, and environmentally hazardous tasks for large sensor networks. In fact, these tasks are practically impossible to perform for embedded sensors, such as those embedded inside structures (bridges, roads, buildings) or airframes. Using micro-power generators (MPGs) to recharge batteries or alternative energy storage devices is an effective solution to this problem. In addition, MPGs can supply energy at higher levels than batteries which allows localized computing and enables new applications such as autonomous wireless sensor networks.
A MPG is a vibration-based apparatus consisting of an oscillator embodied by an inertial mass 4 attached to a spring 2, and of a mechanical damper 3 (FIG. 1). The spring 2 is attached to a housing 6, which is itself rigidly attached to a surface of a host body that experiences ground vibrations (typically a bridge, a car, or a bicycle). As a result, the housing 6 moves with the host body, and the inertial mass 4 moves with respect to the housing 6. A transducer 1 drains some of the apparent kinetic energy from the relative displacement of the inertial mass 4 and converts it to electric energy. The electric energy is routed through electric connectors 11 to a power conditioning circuit 22, and then to a storage device or an electric load 5.
MPGs are designed differently depending on their transduction mechanism. Common transduction mechanisms used in vibration-based MPG include electromagnetic, electrostatic, and piezoelectric mechanisms. For simplicity, similar elements of the MPGs described below with respect to FIGS. 2 to 4 have been labelled with the same reference numerals and will only be described once.
A typical electromagnetic MPG, such as shown in FIGS. 2a and 2b, consists of an inertial mass 4 and a coil 10 supported by a cantilever beam 8 attached to a wall or base 9 of the housing 6 (shown in FIG. 1) and moving in a magnetic field. The coil 10 and magnetic field combination constitutes the transduction mechanism; it ensures the drain of electric energy. The coil 10 is made of a large number of turns of a small gage copper wire or alternatively another conducting non-magnetic material. A system of four permanent magnets 13 maintains the magnetic field in the air gap. The magnets 13 are attached to the inside of a yoke 12 made of steel (or another ferromagnetic material) to increase the flux density in the air gap. Different arrangements of magnets 13 and support structures made of a magnetic material can also be used to maintain a high flux density magnetic field in the air gap (portion in phantom). The relative motion of the inertial mass 4 with respect to the housing 6 causes the beam 8 and the coil 10 it carries to oscillate in the air gap. As a result, an electric current flows through the coil 10 which dampens the relative motion of the beam 8-mass 4 system (or oscillator). Alternatively, the yoke 12 and magnets 13 can act as an inertial mass 4 carried by the beam 8 and move with respect to a fixed coil 10. In either case, the relative motion of the beam 8-mass 4 system is damped (energy is extracted) by the current flowing through the coil 10.
In a typical electrostatic MPG, such as shown in FIGS. 3a and 3b, the transducer consists of a variable capacitor supported by the beam 8. A voltage source (not shown) maintains a potential difference between the capacitor plates 14. The relative motion of the inertial mass 4 with respect to the housing 6 causes the beam 8 and the capacitor plate 14 it carries, to oscillate in the air gap. As a result, the capacitance of the capacitor changes, electric current flows through the circuit connected to the capacitor, and a capacitive force dampens the relative motion of the beam 8-mass 4 system. Alternatively, the capacitor plates 14 can be carried on flexible structures that move relative to the housing 6 which allow them to move with respect to each other. In either case, the relative motion of the beam 8-mass 4 system is damped (energy is extracted) by a capacitive force opposing the motion.
In a typical piezoelectric MPG, such as shown in FIGS. 4a and 4b, the transducer consists of a piezoelectric patch 15 attached to the beam 8. The relative motion of the inertial mass 4 with respect to the housing 6 creates stresses in the beam 8 and the piezoelectric patch 15. The patch 15 transforms this stress to a potential difference between the top and bottom sides of the patch 15. Alternatively, piezoelectric patches 15 can be attached on either side of the cantilever beam 8. In either case, the relative motion of the beam 8-mass 4 system is damped (energy is extracted) by a piezoelectric force opposing the motion.
All these embodiments use the motion of a linear oscillator (the beam 8-mass 4 system) generated by environmental vibrations to create electric energy. Regardless of the transduction mechanisms, the collected energy can be maximized by minimizing dissipation in the mechanical oscillator and parasitic losses in electric circuits, or by maximizing the inertial mass of the MPG to increase the input kinetic energy.
The usability of vibration-based MPGs is severely limited by the random nature of environmental vibrations. Vibration-based MPGs are tuned to harvest energy within a narrow frequency band in the neighborhood of a natural frequency of the oscillator (MPG bandwidth). Outside this band, the output power is too low to be conditioned and utilized. This limitation is exacerbated by the fact that MPGs are also designed to minimize energy dissipation, further narrowing the MPG bandwidth. On the other hand, vibrations in most environments are random and wideband. As a result, vibration-based MPGs can only harvest energy for a relatively limited fraction of time, which imposes excessive constraints on their usability.
Therefore, there is a need for a MPG which would increase the amount of collected energy by increasing the bandwidth of vibration frequencies that can be harvested.